A pharmacist needs to fill an order for a 3% lidocaine topical cream. However, only 2% and 4% concentrations are in Stock. How much of the 4% concentration should be mixed with 30 g of the 2% concentration to get a 3% concentration?

X = grams of 4% cream

0.04x + 0.02 (30) = 0.03 (30+x)

0.04x + 0.6 = 0.9 + 0.03x

0.04x - 0.03x = 0.9 - 0.6

0.01x = 0.3

x = 0.3/0.01

x = 30 grams

3% concentration is exactly inbetween 2%-4% so it makes sense that a 1/2 and 1/2 mixture of both would give 3% concentration.